1. Field of the Invention
This invention relates to a white balance adjustment device for a color image sensing apparatus.
2. Description of the Related Art
Among the known color image sensing apparatuses, an automatic tracking type apparatus is arranged to measure colors by means of an external sensor.
FIG. 3 of the accompanying drawings shows in a block diagram the arrangement of the conventional white balance adjustment device. Referring to FIG. 3, a colorimetric sensor 1 is arranged to detect the red, green and blue components of the light of a light source and to perform photo-electric conversion. A logarithmic compression circuit 2 is arranged to logarithmically compress the red, green and blue components of the light of the light source obtained by the colorimetric sensor 1. A differential circuit 3 is arranged to perform a subtracting operation on the outputs of the logarithmic compression circuit 2. An image sensing system 5 is arranged to photo-electrically convert the light of an object to be photographed and produces red (R), green (G) and blue (B) signal outputs. An R signal amplifier 6 and a B signal amplifier 7 are arranged to amplify the R and B signal outputs of the image sensing system 5 respectively. A signal processing system 8 is arranged to perform a signal processing action to obtain a predetermined signal. A constant voltage output circuit 12 is arranged to produce a constant voltage for controlling the R signal amplifier 6 and the B signal amplifier 7 by applying it to them in place of the outputs of the colorimetric sensor 1 when the light incident on the colorimetric sensor 1 is insufficient. A switch 13 is arranged to switch the supply of the outputs of the colorimetric sensor 1 over to the outputs of the constant voltage output circuit 12 and vice versa.
In the device of FIG. 3, the colorimetric sensor 1 produces signals SR, SG and SB which respectively correspond to the red, green and blue components of the light of the light source. The signals SR, SG and SB are logarithmically compressed by the logarithmic compression circuit 2. The circuit 2 produces outputs log SR, log SG and log SB, which are supplied to the differential circuit 3. The differential circuit 3 obtains a difference between the logarithmic values of R (red) and G (green) and a difference between those of B (blue) and G. The differential circuit 3 converts these differences into outputs log (SR/SG) and log (SG/SB) which are control voltages for the R signal amplifier 6 and the B signal amplifier 7. The amplifiers 6 and 7 perform white balance adjustment by amplifying the sensed image signals output from the image sensing system 5 according to the above-stated control voltages.
In this instance, if the illuminance of the ambient light is low, the error components of the outputs of the colorimetric sensor 1 and the logarightmic compression circuit 2 are amplified. As a result, the white balance adjustment becomes inadequate. In view of this, the conventional device such as the one shown in FIG. 3 is arranged to detect the value log SG as the illuminance of the ambient light. When the detection output log SG is lower than a given reference level, the control is performed by switching the control voltages obtained from the colorimetric sensor 1, the logarithmic compression circuit 2 and the differential circuit 3 over to the outputs of the constant voltage output circuit 12 by means of the switch 13. In this case, the constant voltage output is set at a value corresponding to light of a low color temperature ( which corresponds to candle or tungsten light). It has been thus attempted to prevent the white balance adjustment from becoming excessively inadequate by the above-stated arrangement in the event of low illuminance.
In accordance with the conventional white balance adjustment device, therefore, the control voltage is fixed to the constant voltage output when the ambient light illuminance detection output becomes lower than a given value and the constant voltage is switched over back to the control voltage when the illuminance level of the ambient light comes to exceed the given value. As a result, the white balance tends to greatly change at the above-stated given value. This makes an uncomfortable picture. Particularly, in a case where the ambient light is widely different from a color temperature defined by the constant voltage, a very large change in white balance occurs conspicuously. Further, such change also tends to occur under a flickering light source such as a fluorescent lamp.
Further, in the apparatus of the above-stated kind, white balance correction control is automatically performed on the basis of information on the image of an object obtained through a lens. Then, in cases where there is no completely white part in the object or where a background occupying a large area is of a uniform color other than white, if the white balance control is performed on the basis of the value of a high luminance part of the object or an average value of the video (chrominance) signal of the whole image, a signal for a part which is not white would be controlled in such a way as to show the part as a white part when the object's image is displayed on a monitor. Therefore, the white balance within the picture plane of the monitor would be lost.
To solve this problem, for example, a table showing a white balance correcting (or control) range may be stored in a white balance controlling microcomputer. The white balance then may be controlled within a correcting range limited by the table. In accordance with this method, the white balance can be effectively controlled, by narrowing the correcting range mostly for magenta and green colors, for an object having no white part and a background of a uniform color other than white, which are difficult for automatic white balance control by the (TTL) method as mentioned above.
However, since the white balance is controlled within the limited correction range, the image sensing apparatus of the above-stated kind has been incapable of adequately controlling the white balance for all the color temperatures of objects, for example, in cases where the object to be photographed is white in a large portion thereof or where the white balance is completely adjusted with a standard white plate or the like before an image sensing operation.